A method and apparatus for the measurement of 
wafer thickness, flatness and the trench depth of any trenches etched thereon using the back surface of the 
wafer to accurately measure the back side of a trench, rendering the trench an effective bump, capable of being measured on the top surface and the bottom surface through a non-contact 
optical instrument that simultaneously measures the 
wavelength of the top surface and bottom surface of the 
wafer, converting the distance between wavelengths to a thickness measurement, using a 
light source that renders the material of which the wafer is composed transparent in that 
wavelength range, i.e., using the near 
infrared region for measuring the thickness and trench depth measurement of wafers made of 
silicon, which is opaque in the visible region and transparent in the near 
infrared region. Thickness and flatness, as well as localized shape, can also be measured using a calibration method that utilizes a pair of optical styli.